1. Field of the Invention
The present invention relates generally to the field of heat transfer devices and more particularly to an apparatus and method for the transfer of heat from the surface of a semiconductor chip package.
2. Art Background
Integrated circuits (ICs) dissipate power during operation, generating heat. To avoid device failure, this heat must be transferred away from the IC at a sufficient rate such that the temperature of the die remains within operating parameters. Modern manufacturing processes allow a more dense population of transistors in the same die space than previously available, leading to ICs that dissipate more heat and require more cooling.
Prior art solutions to cooling ICs are typically inefficient, but they are sufficient for cooling low power devices. As the power dissipation of the IC increases, the inefficiencies of the prior art cooling systems are magnified. Nonetheless, if there were no other considerations in computer system design, the prior art cooling systems could be adapted to provide an adequate solution. However, ICs that are components in a computer system are typically contained within an enclosure that is a compromise of several important goals.
The most important goal of computer enclosure design is to provide adequate cooling to the components. Typically, the ICs are cooled by combining conduction and convection techniques for heat transfer. For example, a heat spreader type of heat sink manufactured of a highly heat conductive material will be coupled to the IC package. Heat is transferred from the smaller surface area of the die to the larger surface area of the heat sink. This is an example of conduction. A fan blows air across the surfaces of the heat sink. Heat is transferred to the air mass, which continues to move away from the heat sink. This is an example of convection or mass transport.
Other goals of enclosure design include minimizing the size of the enclosure and minimizing the acoustic signature of the system. When high power ICs are used by the system, at least one of these goals most likely will be sacrificed if the typical prior art cooling solutions are to used. One method for providing sufficient cooling using prior art techniques is to increase the mass flow of air across the heat sinks. Unfortunately, this requires either installing a larger fan or increasing the operating voltage of the fan that is already installed. Neither of these solutions is desirable because installing a larger fan increases the size of the enclosure and increasing the operating voltage of the fan increases the acoustic signature of the computer system.
If the size and operating voltage of the fan are to remain unchanged when high power ICs are used, the inefficiencies of the prior art heat transfer systems must be addressed. First, the heat spreader/heat sinks of the prior art do not exhibit isothermal behavior across the entire surface of the heat sink. This is because there are some thermal losses in the conductive material of the heat sink. The temperature gradient created by these thermal losses results in thermal overhead that requires greater mass flow of air across the heat sink. Second, the heat capacity of air is quite low. Therefore, a heat transfer medium other than air is preferred.
Accordingly, as will be described, a first embodiment of the present invention is an apparatus that uses a heat pipe having minimal thermal overhead and approximately isothermal behavior as a heat sink. A second embodiment of the present invention is a heat transfer system using a circulating heat transfer liquid. As will be shown, the method and apparatus of the present invention provide sufficient cooling for high power ICs with a minimum impact on the goals of minimizing enclosure size and minimizing the acoustic signature of the system.